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| Digital orthophotos require a digital terrain model to rectify the raw photo images into the photo map. Displacement of the image occurs through a number of phenomena: the attitude of the aircraft when the image was taken, the normal radial distortion from the centre of the lens outwards, and the displacements due to differences in height of the land features below. The first two distortions can be solved using various techniques of photogrammetry, ground control and, sometimes, inertial systems. However, the latter requires knowledge of the changes in elevation of the ground. These can be acquired using laborious ground surveys (except in wooded areas), or photogrammetric compilation, which again can be time consuming, and impossible in some tree covered areas. LIDAR data are acquired remarkably quickly and, even in fairly dense vegetation, can penetrate to the ground in places where other methods will not. |
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